1. Field of the Invention
The present invention relates to a high tensile strength steel sheet excellent in processibility (stretch-flanging property and total elongation), and relates to technique for improving a TRIP (TRansformation Induced Plasticity) steel sheet.
2. Description of the Related Art
Steel sheets used for press molding in automobiles and industrial machines are required to have both of excellent strength and processibility, and such property requirements have been recently increased gradually. In order to respond to such demands, recently, TRIP steel sheets have been attractive and paid attention. TRIP steel sheets have a retained austenite, and the retained austenite (γR) is induced—transformed into martensite by a stress, and a great elongation is exhibited when processed and deformed at a temperature of a martensite transformation initiating temperature (Ms point) or higher. For example, TRIP—type composite steels (PF steel) comprising polygonal ferrite+bainite+retained austenite, and TRIP—type bainite steels (BF steel) comprising bainitic ferrite+retained austenite+martensite are known. However, the PF steel is inferior in stretch-flanging property, and the BF steel is excellent in stretch-flanging property, but has a defect that elongation is small.
Then, in order to provide a steel sheet which maintains excellent in balance between strength and elongation due to the retained austenite and also excellent in moldability such as stretch-flanging property (hole enlarging property), various studies have been performed. For example, the following Patent Publications 1 to 4 teach that steel sheets comprising a matrix microstructure of tempered martensite, tempered bainite and the like, and also a second phase microstructure of retained austenite, are excellent in all of strength, elongation and stretch-flanging property (U.S. Patent Application Publication No.: US-2004-0074575-A1). These steel sheets are manufactured by, for example, steps of adjusting a cooling rate after hot rolling to introduce a martensite and a bainite, performing cold rolling, and then cooling the plate from a ferrite-austenite two phase region temperature in a specific pattern to produce retained austenite.